Mitigation Of Salt Accumulation In Osmotic Membrane Bioreactors Using Ion Exchange Resins

Osmotic membrane bioreactor（OMBR）is an emerging technology with great potentials for wastewater treatment and resource utilization.Nevertheless,salt build-up is still one of main obstacles of the OMBR development,and it is a hot issue at present.In this paper,an IERs-OMBR-membrane distillation（MD）hybrid system was proposed based on the study of salinity controlling with mixed ion exchange resins as an adsorbent.Compared with a conventional OMBR-MD system,FO water flux,sewage treatment efficiencies,properties of activated sludge and membrane fouling were investigated,and the addition of IERs obviously alleviated the salt accumulation and membrane fouling in OMBR.Firstly,the static adsorption of sodium chloride by mixed IERs was discussed.The experimental results showed that the combination of gel strong base anion exchange resins（201×7）and large pore strong acid cation exchange resins（D001）had higher desalting rate and adsorption capacity than the other types of the mixed IERs.The optimum static adsorption conditions were as follows:the mixing ratio of anion and cation resins was 3:1,the dosage of the mixed IERs was 16 g/L,and the adsorption equilibrium time was 120min.After five cycles of regeneration with 0.7 mol/L Na OH and 0.7 mol/L HCl,the desalting efficiency was still 81.35%.The adsorption processes of Cl^-and Na⁺by 201×7and D001 both conformed with the Langmuir isothermal adsorption model,quasi-second-order kinetics model and three-stage intra-particle diffusion model.The results of FTIR and SEM illustrated that physical and chemical adsorption existed in the adsorption process of 201×7 and D001.In addition,coexisting ions and organic compounds might inhibit the desalting efficiency.The performances of the IERs-OMBR-MD and OMBR-MD in treating domestic sewage were compared.The results showed that the mixed IERs dosage of 8g/L could keep a conductivity of 6.87 m S/cm in the bioreactor during 30 days of operation,which was30%lower than the conductivity in a traditional OMBR-MD without adding IERs.A water flux difference of about 58%was found between the OMBR-MD system（1.135 L/（m²·h））and the IERs-OMBR-MD system（2.675 L/（m²·h））.The two systems both had high removal efficiencies of organic matters and nutrients,and the MD effluent could meet the standard of"The reuse of urban recycling water-Water quality standard for urban miscellaneous use（GB/T 18920-2020）".The addition of the IERs effectively mitigated the salt accumulation in the OMBR.The MLVSS/MLSS and SOUR in the IERs-OMBR-MD system were higher than those in the OMBR-MD system,and the SMP、EPS and p H values were lower than those in the OMBR-MD system.The ORF model was used to calculate the membrane fouling resistance of two systems.After 30 days of operation,the FO membrane fouling resistance of the IERs-OMBR-MD and OMBR-MD systems was 65.038×10¹⁴m^-1and 116.039×10¹⁴m^-1,respectively.SEM and FTIR were used to characterize the FO membrane before and after conta mination.It was found that the FO membrane surface of the OMBR-MD system was covered with dense filter cake layer,while the FO membrane surface of the IERs-OMBR-MD system was relatively smooth,and the membrane surface structure was still clear.The FTIR results showed that the main organic matters on the FO membrane surface in two systems were protein and polysaccharide.The concentrations of polysaccharide,protein and TOC on the FO membrane surface in the IERs-OMBR-MD system were 32.5 mg/L,30.0 mg/L and 81.8 mg/L,respectively,and those in the OMBR-MD system were 55.0 mg/L,37.5 mg/L and 111.3 mg/L,respectively.The XRD results showed that the inorganic matters on the FO membrane surface were mainly Na Cl and Ca CO₃.When TDS ranged from 100 mg/L to 400 mg/L,the static desalting efficiencies of the regenerated mixed IERs were still above 95%.SEM was also used to observe the conta minated IERs,and it was found that the resins surface was covered with a layer of dense pollutants.The FTIR results showed that the IERs could adsorb a small amount of organic matters such as proteins and polysaccharides.